The present invention relates to an image forming apparatus provided with a developing device based on an electrophotographic method, and in particular, to an image forming apparatus provided with a developing device using a magnetic brush developing method in which development is carried out by the use of two-component developer composed of a magnetic carrier and a non-magnetic toner.
In a conventional magnetic brush developing method using a two-component developer composed of a magnetic carrier and a non-magnetic toner, from the necessity of securing a good developability causing no deficient density, it has been required to supply a sufficient amount of the developer within a range not to produce a problem of the spilling of the developer or the adhering of the carrier particles, in the development region where the magnetic brush layer composed of the developer formed on the outer circumferential surface of a developer carrying member (also called a developing sleeve) is in contact with or close to the outer circumferential surface of an image carrying member (also called a photoreceptor).
For this reason, it has been generally used a developing device using a value near 100% as the coverage ratio (to be described later) of a magnetic brush layer to cover the outer circumferential surface of a developer carrying member.
Further, for the purpose of securing the image density of solid areas, it has been used a method in which the speed ratio of the developer carrying member to the image carrying member was set at a high value, and developer of an amount as much as possible was made to pass the development region while a latent image was passing the region.
However, in recent years, requirement for obtaining a color image and a higher image quality has been enhanced, and now it is necessary a technology for obtaining a toner image which is uniform even in a half-tone area, that is, in a weak electric field area.
However, in a conventional magnetic brush developing method, when a half-tone image is formed, it frequently occurred that image defects such as unevenness of density and scratches in the edge portions were produced.
These image defects are phenomena which occur because a toner image once formed on the photoreceptor drum is rubbed and disturbed by the magnetic brush layer under the somewhat excessive rubbing action of the magnetic brush on the photoreceptor surface due to the difference in the moving speed between the developing sleeve and the photoreceptor drum.
The probability of the occurrence of an image defect is determined by the relation between the following factors: the state of coverage of the outer circumferential surface of the developer sleeve by the developer, which relates to how much developer exists on the outer circumferential surface of the developing sleeve in the development region, the peripheral speed ratio (Vr) of the outer circumferential surface of the developing sleeve to the photoreceptor drum surface, and the ratio (Hr) of the height of heads of the magnetic brush layer (Hh) in the development region (hereinafter referred to as xe2x80x9cthe height of the magnetic brush layerxe2x80x9d simply) to the spacing of the outer circumferential surface of the developing sleeve (Hd) to the outer circumferential surface of the photoreceptor drum in the development region.
That is, if too great account is made of the securing of the density in solid areas, and the peripheral speed ratio (Vr) of the outer circumferential surface of the developing sleeve to the photoreceptor drum, the height ratio of the magnetic brush layer (Hr), and the above-mentioned coverage ratio of the magnetic brush layer are made larger excessively, image defects such as unevenness of the density in a half-tone image and scratches in the edge portions tend to occur.
The inventors of the present invention noticed that, for the toner quantity contributing development, not the developer quantity on the surface of the developing sleeve but the quantity of toner that is brought in a state where the toner particles are capable of facing the latent image on the photoreceptor drum is important. That is, it is important how large an area the magnetic brush layer covers of the surface of the developing sleeve in the development region.
Further, the rubbing action by the magnetic brush layer to disturb a toner image, as described in the above, is influenced by the peripheral speed ratio Vr of the developing sleeve to the photoreceptor drum, and also by the height ratio Hr of the magnetic brush layer.
It is an object of the present invention, in order to solve the above-mentioned problem, to provide an image forming apparatus capable of forming an image securing a high density in solid black areas and having no unevenness of density and no scratch in the edge portions in a half-tone image, by the values of the spacing of the developer carrying member to the image carrying member, the peripheral speed ratio of the developer carrying member to the image carrying member, height of the magnetic brush layer, and the coverage ratio of the magnetic brush layer being made to have a relation with one another.
The object of the present invention can be accomplished by any one of the structures (1) and (2) described below.
(1) An image forming apparatus comprising an image carrying member for forming an electrostatic latent image, a developing device provided with a developer carrying member for carrying a two-component developer containing non-magnetic toner particles and magnetic carrier particles, a magnetic field generating means provided inside said developer carrying member fixedly, and a developer layer regulating member for regulating the height of a magnetic brush layer formed of the two-component developer on said developer carrying member, and a bias voltage applying means for applying a bias voltage composed of a direct-current bias voltage and an alternate-current bias voltage superposed to said developer carrying member, wherein with the peripheral speed ratio of the outer circumferential surface of said developer carrying member to the outer circumferential surface of said image carrying member denoted by Vr, the spacing of the outer circumferential surface of said developer carrying member to the outer circumferential surface of said image carrying member denoted by Hd, the average value of the height of said magnetic brush layer denoted by Hh, the ratio of said average value of the height Hh to said spacing Hd denoted by Hr, and the coverage ratio of the area of said magnetic brush layer to cover the outer circumferential surface of said developer carrying member to the outer circumferential surface of said developer carrying member denoted by xcex1%, the relation connecting Vr, Hr, and xcex1 is expressed by the following inequality:
1xe2x89xa6(xcex1/100)xc3x97Vrxc3x97Hr1/3 less than 3.5. 
(2) An image forming apparatus as set forth in the structure (1) characterized by the aforesaid two-component developer having a toner concentration expressed by weight percent of 3% to 10% to the developer composed of magnetic carrier particles having a volume-average particle diameter of 30 xcexcm to 60 xcexcm and a saturation magnetization of 30 emu/g to 60 emu/g and non magnetic toner particles having a volume-average particle diameter of 4 xcexcm to 11 xcexcm mixed.